Flat-model liquid crystal display devices and PDP devices have largely replaced CRT devices in the field of display devices such as computers and televisions. Of the flat-model display devices, PDP devices present sharper images because pixels themselves emit light. PDP devices are therefore frequently used in place of liquid crystal display devices.
It is necessary with PDP devices that an EMI filter is bonded to the front to shield electromagnetic waves generated by the emission of light from pixels. Because the surface of PDP devices becomes hot during operation and cools after shutdown, adhesives that are applied to PDP are required to have little change in bond strength by thermal cycles and maintain good bonding over long periods. EMI filters are sometimes applied to the plasma display panel directly, and misalignment or filter defects may require that the filters are removed. It is therefore necessary that the EMI filters are removable clean without residual adhesive.
Regarding pressure-sensitive adhesive films for the bonding of electronic displays including plasma display panels, Patent Document 1 (JP-A-2002-107507) discloses a pressure-sensitive adhesive film wherein the pressure-sensitive adhesive contains a (meth)acrylic resin having a weight average molecular weight (Mw) of 500,000 to 2,000,000 and a glass transition temperature (Tg) of not more than −20° C. and an acrylic resin having a weight average molecular weight (Mw) of 1000 to 100,000 and a glass transition temperature (Tg) of not less than 50° C. in a weight ratio of 97-80:3-20 and further contains a crosslinking agent (C).
By containing the high molecular weight (meth)acrylic resin and the low molecular weight (meth)acrylic resin in the specific ratio, this pressure-sensitive adhesive achieves relatively good properties for the bonding of electronic displays. However, lifting or separation often arises from the pressure-sensitive adhesive layer during long use accompanied by thermal cycles such as in PDP devices wherein the temperature is greatly changed by the operation.
Patent Document 2 (JP-A-2004-256599) discloses a pressure-sensitive adhesive composition and a pressure-sensitive adhesive optical sheet with the composition wherein the composition contains a (meth)acrylate copolymer having a weight average molecular weight (Mw) of 400,000 to 2,500,000 and a glass transition temperature of not more than 0° C. and a (meth)acrylate copolymer having a weight average molecular weight (Mw) of 1000 to 50,000 and a glass transition temperature (Tg) of 80 to 230° C. in a ratio of 100:5-200 and further contains a crosslinking agent.
The pressure-sensitive adhesive composition and the pressure-sensitive adhesive optical sheet achieve excellent properties as optical members by the crosslinking of the high molecular weight (meth)acrylate copolymer and the low molecular weight (meth)acrylate copolymer that are contained in the specific ratio. However, lifting or separation often arises from the pressure-sensitive adhesive layer during long use accompanied by thermal cycles such as in PDP devices wherein the temperature is greatly changed by the operation.
Patent Document 3 (JP-A-H10-310754) discloses a pressure-sensitive adhesive composition that contains 100 parts by weight of an alkyl (meth)acrylate copolymer (1) having a weight average molecular weight (Mw) of not less than 800,000 and 1 to 40 parts by weight of an alkyl acrylate copolymer (2) having a weight average molecular weight of not more than 100,000. However, Patent Document 3 does not have a technical idea that the high molecular weight alkyl (meth)acrylate copolymer (1) may be an alkyl (meth)acrylate copolymer having a weight average molecular weight (Mw) of less than 800,000.
Patent Document 4 (JP-A-H5-70752) describes that improved heat resistance and cold resistance are obtained in the production of acrylic pressure-sensitive adhesive compositions and pressure-sensitive adhesive tapes, labels or sheets by controlling the acryl polymer's molecular weight distribution (Mw/Mn) to not more than 4. Further, it describes that heat resistance is enhanced by increasing the glass transition temperature. Patent Document 4 teaches that the products are used in the electric and electronic fields but does not mention use in the optical field such as PDP.    Patent Document 1: JP-A-2002-107507    Patent Document 2: JP-A-2004-256599    Patent Document 3: JP-A-H10-310754    Patent Document 4: JP-A-H5-70752